This invention relates to the detection of receivers by transmitters coupled to a transmission link. A transmitter coupled to a link may be required to detect whether a receiver is coupled to the link before starting transmission on the link. The detection may serve a variety of purposes, including reducing extraneous transmissions which may increase transmitter power consumption or the production of electromagnetic interference, for example. The detection may also be required by new transmission protocols, such as the PCI-Express standard.
The receiver detection may be based on measuring changes in the electrical characteristics of the link when a receiver is coupled or not coupled to the link. These characteristics may be measured from the output of the transmitter, and may include the input impedance of the link. While the presence of a receiver on the link may change the characteristics of the link, the changes may be relatively small and difficult to measure. Furthermore, variations in the electrical characteristics of the link, such as variations in link resistance (or impedance) and capacitance, may make it harder to detect changes due exclusively to the presence of a receiver. Similarly, variations in the electrical characteristics of the receiver, such as receiver input impedance, may complicate detection. Finally, variations in the receiver detection circuitry in the transmitter may compromise the accuracy of the detection process. These variations may include variations in power supply level, transmitter output impedance, as well as parasitic capacitances or impedances of transmitter output leads, comparator offset, and other factors that may affect the accuracy and function of the detection circuitry and methods. A further challenge is posed by low power supply levels, such as 1.2V supplies, which limit the voltage range useable for receiver detection.
Receiver detection circuitry and methods that are less sensitive to variations in transmitter, link, and receiver characteristics, may improve the accuracy of receiver detection.